The present invention relates to a magnetic recording medium and to a method of manufacturing the same. The magnetic recording medium is installed in various magnetic recording devices.
Perpendicular magnetic recording methods, which have been studied for many years as technology to achieve high recording densities in magnetic recording, have recently been commercialized. In such methods, the recording magnetization is made perpendicular to the plane of the recording medium; henceforth perpendicular magnetic recording is expected to replace conventional longitudinal recording methods, in which the recording magnetization is parallel to the plane. A perpendicular magnetic recording medium (or more concisely, perpendicular medium) used in perpendicular magnetic recording mainly comprises a magnetic recording layer of a hard magnetic material; an underlayer to orient the recording magnetization in the magnetic recording layer in the perpendicular direction; a protective layer to protect the surface of the magnetic recording layer; and a backing layer of soft magnetic material, which serves to concentrate the magnetic flux generated by the magnetic head which is used to record in the recording layer.
Guidelines for design of media to further raise recording densities include promotion of magnetic separation of the crystal particles comprised by the magnetic recording layer, to reduce the units of magnetization inversion. Normally, the film thickness of the magnetic recording layer is constant, so that if the units of magnetization inversion are made smaller, the demagnetizing field acting on the magnetic recording layer is smaller. As a result, the magnetic switching field of the magnetic recording layer is increased. In this way, when considered simply in terms of the shape of magnetization inversion units, raising the recording density can be regarded as requiring a stronger write magnetic field. However, the magnitude of the demagnetizing field also differs depending on the magnitude of the magnetization of the particles themselves, and so the material composition, film thickness, and other parameters are optimized such that the switching field is less than the magnetic field generated by the head.
However, as the recording density rises, in order to make particles finer, the magnetic anisotropy energy (Ku) must be increased. If this energy is smaller than the demagnetizing field energy, it becomes difficult to stably maintain perpendicular magnetization. However, increasing Ku again entails increasing the write magnetic field. Thus a method is sought for improving the thermal stability of the magnetic recording medium and the electromagnetic transducing characteristics, without increasing the write magnetic field.
To address this problem, methods of either dividing the magnetic recording layer into two or more layers and varying the composition during film deposition, or of inserting a nonmagnetic layer between the divided magnetic recording layers, have been proposed (see for example Japanese Patent Laid-open No. 2003-157516 (corresponding to U.S. Patent Publication No. 2003096127 A1)). In this Japanese Patent Laid-open No. 2003-157516 (corresponding to U.S. Patent Publication No. 2003096127 A1), it is reported that by dividing the magnetic recording layer and interrupting epitaxial growth, magnetic recording medium noise can be reduced while maintaining a volume per magnetization inversion unit necessary to improve thermal stability.
Further, in Japanese Patent Laid-open No. 2004-39033 (corresponding to U.S. Patent Publication No. 2004053078 A1), a method is proposed in which, by applying antiferromagnetic coupling employed in in-plane magnetic recording media to perpendicular media, reverse domain noise can be reduced, and resistance to thermal fluctuations can be improved. In Japanese Patent Laid-open No. 2006-48900 (corresponding to U.S. Patent Publication No. 2006177703 A1), a method is proposed in which a coupling layer is inserted between two magnetic layers, to ferromagnetically couple the two magnetic layers. By using the above methods, increases in the write magnetic field can be suppressed, that is, ease of writing can be secured.
However, if ease of writing is increased too much, the problem of “adjacent track erasure” prominently appears. This is a problem in which the head recording field exerts an effect extending to tracks adjacent to the track which is originally to be recorded, so that signals are overwritten, that is, erased. Measures on the medium side to address this problem include a method, such as for example described in IEEE Transactions on Magnetics (IEEE Trans. Magn.) B. R. Acharya et al., Vol. 40, No. 4, page 2383 to 2385 (2004), to control the structure of the soft magnetic backing layer. The anisotropy magnetic field of the backing layer is increased, and this tends to sacrifice ease of writing. Hence similarly to the magnetic recording layer, it is anticipated that there will ultimately be a trade-off.
As a novel media-related measure to address such problems, it has been proposed that media be formed with write tracks enclosed between nonmagnetic members, or that in other ways tracks be formed physically in advance. In contrast with conventional “continuous-film media”, such media is known as “discrete-track media”. By this means, both edges in the track width direction are made nonmagnetic, so that spreading during writing is suppressed, and the problem of adjacent track erasure can be alleviated; in addition, there is the further advantage that noise from track edges can be reduced. These effects are expected to appear as the track density is raised.
However, methods to fabricate such media are more complex than in the prior art. For example, in Japanese Patent Laid-open No. 9-97419 (corresponding to U.S. Pat. No. 6,014,296), micromachining techniques such as are used in semiconductor manufacturing are employed to perform etching of a substrate or magnetic film through a mask pattern to form tracks. Specifically a method is described in which “1) A conventional film deposition method is used to deposit films up to a magnetic film on a substrate. 2) A resist film is applied onto this. 3) A pattern is drawn in the resist. 4) The pattern portion is etched, to form a relief pattern in the magnetic film (relief machining). 5) Depressed portions are filled with nonmagnetic members. 6) Flattening is performed. 7) A protective film is deposited on top.”
Also, as for example disclosed in Japanese Patent Laid-open No. 2002-288813, a method has been disclosed in which technology is employed to partially implant ions into a magnetic film to render the implanted portions nonmagnetic, so that tracks are formed by means of nonmagnetic portions/magnetic portions/nonmagnetic portions. This method has the advantages that a flattening technique is not required and only comparatively few manufacturing processes are required. Further, in Japanese translation of PCT application No. 2003-536199, formation of so-called flat patterned media is proposed, in which, by performing partial ion implantation in a coupling layer of an exchange-coupled control layer inserted between magnetic layers, ferromagnetic coupled regions and antiferromagnetic coupled regions are formed, and the magnetizations in upper and lower layers cancel in the antiferromagnetic portions, so that signal output from the ferromagnetic coupled portions is reinforced.
In continuous film media such as described in Japanese Patent Laid-open No. 2003-157516 (corresponding to U.S. Patent Publication No. 2003096127 A1), Japanese Patent Laid-open No. 2004-39033 (corresponding to U.S. Patent Publication No. 2004053078 A1), and Japanese Patent Laid-open No. 2006-48900 (corresponding to U.S. Patent Publication No. 2006177703 A1), as track densities are increased, the problem of adjacent track erasure becomes prominent, and moreover track edge noise can no longer be ignored. That is, raising the track density becomes difficult.
On the other hand, in the case of discrete media proposed in the past such as in Japanese Patent Laid-open No. 9-97419 (corresponding to U.S. Pat. No. 6,014,296), the method of manufacture is attended by major problems. In general, the magnetic recording layer film thickness is designed to be 10 nm or greater, so that when head flying stability is considered, methods employing micromachining techniques necessitate the use of flattening. Unevenness in the thickness of the magnetic recording layer is directly related to fluctuations in signal strength, and so considerable precision is required in the flattening process. However, when CMP (chemical-mechanical polishing) is used for flattening, this problem becomes particularly prominent, whereas when dry etching is employed, comparatively uniform machining is possible, but long lengths of time are required, posing problems from the standpoint of manufacturing. And, when using ion implantation methods such as in Japanese Patent Laid-open No. 2002-288813 and in Japanese translation of PCT application No. 2003-536199, flattening techniques are not necessary, but it is difficult to control the spreading of implanted ions, the effect of backscattering, and other factors. In particular, when using the method employing ion implantation into a coupling layer of Japanese translation of PCT application No. 2003-536199, the effect of backscattering causes changes in the magnetic characteristics of the magnetic layer immediately below, and this means degradation of the characteristics of the recording layer which is crucial for information recording. In addition, the crystal structure in the coupling layer is disordered, so that there is the drawback that epitaxial growth of another magnetic layer formed directly above is impeded. This also means that the functions of the recording layer are degraded. Moreover, the methods of Japanese Patent Laid-open No. 2002-288813 and Japanese translation of PCT application No. 2003-536199 make no contributions with respect to improvement of the media write performance.
In the magnetic recording medium described in Japanese translation of PCT application No. 2003-536199, a spacer film is patterned into a first region, having thickness sufficient to guide magnetic flux through the spacer film and a second ferromagnetic film so as to be antiferromagnetically exchange-coupled with a first ferromagnetic film, and a second region, in which the first and second ferromagnetic films are not antiferromagnetically exchange-coupled; by this means, the second region effectively causes stronger magnetic fields to occur over the magnetic layer than the magnetic fields from the first region.
However, in Japanese translation of PCT application No. 2003-536199, in addition to the problem with manufacturing described above, because the recording method is an in-plane recording method, there is a problem from the standpoint of higher densities. That is, in one layer among the two layers, there occur portions in which the magnetization directions are not necessarily opposing between the first region and the second region, and so as bit sizes are reduced, thermal stability deteriorates. This means a return to the problems that were the reason for replacement of in-plane recording with perpendicular recording methods.